In 1990, we were referred a 4-year old child with a new chronic disorder we have termed autoimmune lymphoproliferative syndrome, or ALPS. The child appeared rather well but for dramatic enlargement of her liver, and all lymphatic chains. An enlarged spleen had been removed. She experienced recurrent hemolytic anemia. Lymphocyte phenotype demonstrated marked expansion of CD4-/CD8- T cells. Several aspects of that original patient suggested a biological relationship to the heritable mouse illness associated with either the lpr or gld genotypes. The mice develop profound lymphoproliferation and expansion of double negative T cells. They also develop one or more autoimmune phenomena including hemolytic anemia, vasculitis, or glomerulitis. Since the original observation of this first child, we have identified affected children and adults in about 30 additional families, some with multiple members. All possess marked splenomegaly and variable lymphadenopathy&#61507; in some cases, quite extreme. Splenectomies have been required in most of the patients to control anemia, thrombocytopenia and neutropenia. The lymphocyte phenotype shows that 1% to 40% of all lymphocytes are CD4-/CD8- and that nearly all double negative cells possess the A/ETCR. This represents a 2- to 1000-fold expansion of the double negative cells. Other than splenectomy, treatments have generally not been effective but for prednisone or intravenous immunoglobulin to aid in control of autoimmune hemolysis. High doses of prednisone do not yield sustained reductions in lymphocyte numbers or lymphadenopathy, nor does recombinant IFN-&#61474; and cyclosporin A. Cytotoxic drugs are sometimes required to control autoimmunity. We have found markedly elevated IL-10 levels in nearly all of the subjects. We are seeking the cellular origin of the IL-10 and the effects of the IL-10 on other immune pathways. Thus far, the work indicates that the IL-10 emanates, at least in part, from the vastly expanded monocyte-macrophage population. Quantitative RT-PCR assays reveal significantly elevated levels of IL-10 mRNA in circulating and tissue lymphocytes. Extensive studies of other cytokines showed slightly elevated circulating TNF-alpha, decreased release from stimulated CD4+DR+T cells of IL-2, IFN-gamma and increased IL-4 and IL-5. Monocyte-macrophages release decreased amounts of IL-12. The cumulative cytokine profile is compatible with the humoral autoimmunity and decreased DTH responses seen in ALPS patients. In vitro studies with human cells and cell lines showed that ALPS is associated with inherited defects in lymphocyte apoptosis. Most patients possess specific functional mutations in the gene encoding Fas, a protein that triggers apoptosis, the programmed death of lymphocytes. Because Fas is critical to lymphocyte apoptosis, the cells persist when no longer appropriate leading to accumulation of cells infiltrating tissues&#61472;including cells with reactivity against self-antigens. Initial studies showed that Fas is expressed on the surfaces of our patients cells, but some of it is abnormal, arising from one mutated copy of the pair of Fas genes. The defective Fas protein that is expressed interferes with the function of the normal Fas protein. We have found Fas mutations responsible for ALPS in all but 4 of our affected families. Curiously, we could identify identical Fas mutations in other family members over multiple generations, but most were healthy. We believe that a second mutation must be introduced from the other parent as well&#61472;as one that renders a clinically significant failure in lymphocyte apoptosis. Also, four patients with ALPS were found to have no Fas mutations, nor were there mutations in the Fas-L. Nonetheless, these cells show impaired apoptosis in vitro. Thus, there must be additional genes that when mutated can lead to ALPS. Preliminary studies implicate a specific MCH-4-mutation as leading to defective apoptosis and ALPS in 2 families. We have been able to use Fas-defective ALPS patent cells to study the role of Fas in HIV-mediated apoptosis. HIV killing of CD4 cells was unimpeded by defective Fas proteins, establishing conclusively, that Fas is not necessary for killing of HIV- infected cells.